1. Field of Invention
The present invention relates to a method of designing photomasks. More particularly, the present invention relates to a method of designing photomasks by adjusting the phase shift and/or the optical transmission of the mask pattern.
2. Description of Related Art
Due to the rapid development of integrated circuit techniques, device miniaturization and integration is a common trend. In the process of fabricating an integrated circuit, photolithographic processes have major influences on the ultimate operating characteristics of the fabricated devices. As the level of integration continues to increase, device dimensions as well as distance of separation between neighboring devices are reduced correspondingly. Consequently, a pattern transferred to a silicon chip using a photolithographic process may result in some deviation. For example, line-ends of a line pattern may be pulled up (reduced) or sharp corners in the pattern may be rounded. The aforementioned deviations have great negative effect on the integrated circuit as dimensions of devices are relatively small or the level of integration is high. If there is a deviated reduction in the line-end when a pattern is transferred to a silicon chip, pattern misalignment may occur and the process window tolerance is decreased.
In order to increase the pattern transfer fidelity and the resolution of photolithographic processes, many technologies, such as, phase shift mask (PSM) and optical proximity correction (OPC), have been developed.
The basic concept of phase shift masks is to add a shifter layer between adjacent apertures of the mask patterns, causing a 180-degree phase shifting of the light. The shifter layer can reverse the phase and induce interference, thus enhancing resolution for the images at the wafer. The shifter layer can be designed with a specific thickness and a refractive index in order to cause a 180-degree phase shift, so that diffraction from adjacent apertures can be cancelled out, thus increasing the exposure resolution and improving uniformity for critical dimensions of the device.
Similarly, to prevent variation of critical dimensions in mask pattern transfer, an optical proximity correction procedure is often applied during photomask fabrication. In general, optical proximity correction includes the addition of serifs to the corners or hammerheads to end edges of the original pattern to prevent the rounding of right-angled corners and the pullback of line-ends.
However, when the critical dimension or the distance between neighboring lines has been reduced to a certain extent, addition of serifs or hammerhead patterns or conventional phase shift masks can no longer avoid the undesirable pullback of line-ends effectively.